This project is aimed at understanding the fundamental mechanisms of antibody-mediated ricin-A-chain cytotoxicity and its potential practical application to cancer chemotherapy. The cell-binding and toxic activities of many plant and bacterial toxins are structurally segregated into separate regions or subunits. These developments led us to produce unique conjugates between the toxic A chain of ricin and several cellsurface reactive antibody carriers. Different methods for establishing a cleavable disulfide bond between the purified A chain and antibody have been utilized so that both univalent and bivalent antibody molecules could be examined. In vitro examinations of these conjugates have shown that the undesirable toxicity associated with whole toxin is avoided, yet a high degree of antibody-directed cytotoxicity is retained. A panel of distinct monoclonal antibodies directed against single loci on human immunoglobulin lambda and kappa light chains, as well as on gamma and delta heavy chains, has been developed. Their reactivity with precise phenotypic markers present on the membranes of various human leukemias, lymphomas, and cell lines of B-cell origin has been demonstrated. We have produced and characterized individual hybridoma lines that produce monoclonal antibodies directed against ricin-A-chain and against ricin-B-chain. Radioimmunoassays for measuring serum levels of ricin-A-chain conjugates have been devised and are being utilized to monitor the pharmacokinetics of the intact cytotoxin and its component parts in rabbits. It has been found that the highly selective cytotoxicity of site-directed ricin-A-chain conjugates can be potentiated by membrane active carboxylic ionophores. Combined use of the two agents results in much faster inactivation of ribosomes and subsequent cell death and lysis. The potency of A chain cytotoxins is correspondingly increased by several orders of magnitude and cells which sparsely express the target antigen or receptor can be killed. We plan to: (1)\continue pharmacology studies of conjugates in rabbits; (2)\study further the transferrin-A-chain-resistant cells to determine the modifications they have undergone; (3)\explore the use of antiricin monoclonal antibodies for delivery of toxin to cells; (4)\link ricin A chain to additional cell-reactive monoclonal antibodies and test their cytotoxic properties; and (5)\examine the cellular internalization of toxins and antibodies. (IT)